It was not immediately clear why the field continued to be shut, but sources said the blockage was in a different place from a pipeline closure that forced Sharara to cease operations between Saturday and Tuesday.

“They open one valve, they close the other,” one of the sources said, adding that negotiations were ongoing to resolve the problem.

Operations at Sharara restarted at least once on Tuesday amid conflicting reports about whether the field had reopened.

Sharara resumed production in December after the end of a two-year pipeline blockade near the west Libyan town of Zintan.

In recent months it has been affected by repeated shutdowns because of protests by armed groups and oil workers in the field, along the pipeline, and at the Zawiya oil terminal.

The field had shut on Saturday after a group blocked valve 17 on the pipeline linking Sharara to Zawiya.

Libya’s National Oil Corp (NOC) said engineers sent to reopen the valve found a gearbox had been stolen, but added that no group had claimed responsibility or made demands.

The NOC runs Sharara in a joint venture with oil companies Repsol, Total, OMV and Statoil.

Production at Sharara is key to a revival in Libya’s oil output, which surged above 1 million bpd in late June, about four times higher than its level last summer.

Libya has been exempted from an OPEC-led push to cut global production and bolster oil prices, and a recovery in the North African country’s output has complicated the bloc’s efforts to curb supply.